Local water slamming impact on sandwich composite hulls

The local water slamming refers to the impact of a part of a ship hull on stationary water for a short duration during which high local pressures occur on the hull. We simulate slamming impact of rigid and deformable hull bottom panels by using the coupled Lagrangian and Eulerian formulation included in the commercial software LS-DYNA. We use the Lagrangian formulation to describe plane-strain deformations of the hull panel and consider geometric nonlinearities. The Eulerian formulation is used to analyze deformations of the water. Deformations of the hull panel and of the water are coupled through the hydrodynamic pressure exerted by water on the hull, and the velocity of particles on the hull wetted surface affecting deformations of the water. The continuity of surface tractions and the inter-penetrability of water into the hull are satisfied by using a penalty method. The computer code is verified by showing that the computed pressure distributions for water slamming on rigid panels agree well with those reported in the literature. The pressure distributions computed for deformable panels are found to differ from those obtained by using a plate theory and Wagner's slamming impact theory. We have also delineated jet flows near the edges of the wetted hull, and studied delamination induced in a sandwich composite panel due to the hydroelastic pressure.     

Local slamming impact of sandwich composite hulls

We develop a hydroelastic model based on a {3, 2}-order sandwich composite panel theory and Wagner’s water impact theory for investigating the fluid–structure interaction during the slamming process. The sandwich panel theory incorporates the transverse shear and the transverse normal deformations of the core, while the face sheets are modeled with the Kirchhoff plate theory. The structural model has been validated with the general purpose finite element code ABAQUS^®. The hydrodynamic model, based on Wagner’s theory, considers hull’s elastic deformations. A numerical procedure to solve the nonlinear system of governing equations, from which both the fluid’s and the structure’s deformations can be simultaneously computed, has been developed and verified. The hydroelastic effect on hull’s deformations and the unsteady slamming load have been delineated. This work advances the state of the art of analyzing hydroelastic deformations of composite hulls subjected to slamming impact.     

Experimental investigation of hydrodynamic loads and pressure distribution during a pyramid water entry

In the maritime environment slamming is a phenomenon known as short duration impact of water on a floating or sailing structure. Slamming loads are local and could induce very high local stresses. This paper reports a series of impact test results and investigate the slamming loads and pressures acting on a square based pyramid. In this study the slamming tests have been conducted at constant velocity impact with a hydraulic high speed shock machine. This specific experimental equipment avoids the deceleration of the structure observed usually during water entry with drop tests. Three velocities of the rigid pyramid have been used (10, 13 and 15 m s⁻¹). Time-histories of local pressures, accelerations and slamming loads were successfully measured. The relationship between the pressure magnitude and the impact velocity is obtained and the spatial distribution of pressures on pyramid sides is characterized. The impact velocity was found to have a negligible influence in predicting the maximum pressure coefficient.     

Stress Field Evolution under Mechanically Simulated Hull Slamming Conditions

The effect of repeated loading from mechanically simulated hull slamming on foam core sandwich composites was investigated utilizing a novel technique that simultaneously measured temperature and displacement while cyclic loading occurred. Thermoelastic Stress Analysis (TSA) and Digital Image Correlation (DIC) techniques were combined using a single infrared camera for characterization of the foam core. Improved stress fields with TSA results were found through deformation compensation. Initial work approximating hull slamming conditions mechanically utilizing a custom device were performed. Mechanically loading offers several benefits over water impact investigations, including easy access to the sample during the slamming event, an unobstructed optical path, and accelerated testing. Evolving stress fields under long-duration, repeated simulated hull slamming loading were observed around a growing delamination crack between the foam core and skin.     

Delamination in sandwich panels due to local water slamming loads

We study delamination in a sandwich panel due to transient finite plane strain elastic deformations caused by local water slamming loads and use the boundary element method to analyze motion of water and the finite element method to determine deformations of the panel. The cohesive zone model is used to study delamination initiation and propagation. The fluid is assumed to be incompressible and inviscid, and undergo irrotational motion. A layer-wise third order shear and normal deformable plate/shell theory is employed to simulate deformations of the panel by considering all geometric nonlinearities (i.e., all non-linear terms in strain–displacement relations) and taking the panel material to be St. Venant–Kirchhoff (i.e., the second Piola–Kirchhoff stress tensor is a linear function of the Green–St. Venant strain tensor). The Rayleigh damping is introduced to account for structural damping that reduces oscillations in the pressure acting on the panel/water interface. Results have been computed for water entry of (i) straight and circular sandwich panels made of Hookean materials with and without consideration of delamination failure, and (ii) flat sandwich panels made of the St. Venant–Kirchhoff materials. The face sheets and the core of sandwich panels are made, respectively, of fiber reinforced composites and soft materials. It is found that for the same entry speed (i) the peak pressure for a curved panel is less than that for a straight panel, (ii) the consideration of geometric nonlinearities significantly increases the peak hydrodynamic pressure, (iii) delamination occurs in mode-II, and (iv) the delamination reduces the hydroelastic pressure acting on the panel surface and hence alters deformations of the panel.     

Higher-order dynamic response of composite sandwich panels with flexible core under simultaneous low-velocity impacts of multiple small masses

Small mass impactors, such as runway debris and hailstones may result in a wave controlled local response, which is essentially independent of boundary conditions. The higher-order impact model of sandwich beams presented by Mijia and Pizhong [Mijia, Y., Pizhong, Q., 2005. Higher-order impact modeling of sandwich structures with flexible core. International Journal of Solids and Structures 42 (10), 5460–5490] is developed and enhanced to impact analysis of sandwich panels with transversely flexible cores. Therefore, an improved fully dynamic higher-order impact theory is developed to analyze the low-velocity impact dynamic of a system which consists of a composite sandwich panel with transversely flexible core and multiple small impactors with small masses. Impacts are assumed to occur normally and simultaneously over the top face-sheet with arbitrary different masses and initial velocities of impactors. The contact forces between the panel and the impactors are treated as the internal forces of the system. First shear deformation theory (FSDT) is used for the face-sheets while three-dimensional elasticity is used for the soft core. The fully dynamic effects of the core layer and the face-sheets are considered in this study. Contact area can be varied with contact duration. The results in multiple mass impacts over sandwich panels that are hitherto not reported in the literature are presented based on proposed improved higher-order sandwich plate theory (IHSAPT). Finally, for the case study of the single mass impact, the numerical results of the analysis have been compared either with the available experimental results or with some theoretical results. As no literature could be found on the impact of multiple impactors over sandwich panels, the present formulation is validated indirectly by comparing the response of two cases of double small masses and single small mass impacts. Also, in order to demonstrate the applicability of the validation, the analytical relation of minimum distance between two impactors is derived based on Olsson’s wave control principle in this paper.     

冰雹撞击下泡沫铝夹芯板的动态响应

在传统单层泡沫夹芯结构的上、下面板之间插入中面板,通过移动中面板的位置,获得了外形尺寸相同、质量相等的5种构型夹芯结构,其上层芯材与芯材总厚度比分别为0:30、10:30、15:30、20:30和30:30。在量纲分析的基础上,应用非线性动力有限元程序LS-DYNA对5种构型夹芯结构进行了冰雹撞击数值分析,研究了中面板位置对夹芯板的能量吸收、能量耗散和动态响应的影响。结果表明:中面板的存在对下层芯材能形成有效的保护;随着中面板位置由上向下移动,夹芯板的抗撞击性能呈现由大到小再增大的态势。数值计算结果对抗冰雹撞击夹芯结构的优化设计具有一定的参考价值。     

Study on the Parameters Influencing the Accuracy and Reproducibility of Dynamic Pressure Measurements at the Surface of a Rigid Body During Water Impact

Water wave slamming is known as one of the most important load which marine constructions encounter. Especially the large and spiky local pressures moving fast over the body surface during a slamming event can be harmful for the structure. Analytical and numerical research on these pressure loads has already been performed, but however, quantitative experimental information necessary for validation of these studies is restricted. This lack in experimental data may originate from the fact that accurate pressure measurements are difficult to perform. This paper investigates the reason why this type of measurements is so difficult by identifying the parameters affecting the pressure recordings during water impact. According to the authors’ knowledge, no other paper is available in the open literature which investigates all these influencing factors. It has been observed that the pressure signal sampling rate, sensor position, water temperature, object surface conditions and water surface conditions all have an effect on the measured pressures. Only by controlling these parameters, accurate and reproducible results are possible. Precautions in order to meet this goal are formulated.     

泡沫铝子弹高速撞击下铝基复合泡沫夹层板的动态响应

应用一级轻气炮驱动泡沫铝弹丸高速撞击加载技术,对实心钢板以及前/后面板为Q235钢板、芯层分别为铝基复合泡沫和普通泡沫铝的夹层板结构,在脉冲载荷作用下的动态力学响应进行实验研究。结果表明:泡沫铝子弹高速撞击靶板可近似模拟爆炸载荷效果;铝基复合泡沫夹层板的变形分为芯层压缩和整体变形两个阶段;与其他靶板相比,铝基复合泡沫夹层板的抗冲击性能最优。基于实验研究,应用LS-DYNA非线性动力有限元软件,对泡沫铝夹层板的动态响应进行数值模拟。结果表明:泡沫铝子弹的长度和初始速度对子弹与夹层板之间的接触作用力影响显著,并且呈线性关系。泡沫芯层强度对等质量及等厚度夹层板的抗冲击性能均有显著影响,夹层板中心挠度对前、后面板的厚度匹配较为敏感,在临界范围内,若背板厚度大于面板厚度,可减小夹层板的最终挠度。夹层板面板宜采用刚度较低、延性好、拉伸破坏应变较大的金属材料。     

Low-velocity impact response of sandwich beams with functionally graded core

The problem of low-speed impact of a one-dimensional sandwich panel by a rigid cylindrical projectile is considered. The core of the sandwich panel is functionally graded such that the density, and hence its stiffness, vary through the thickness. The problem is a combination of static contact problem and dynamic response of the sandwich panel obtained via a simple nonlinear spring-mass model (quasi-static approximation). The variation of core Young’s modulus is represented by a polynomial in the thickness coordinate, but the Poisson’s ratio is kept constant. The two-dimensional elasticity equations for the plane sandwich structure are solved using a combination of Fourier series and Galerkin method. The contact problem is solved using the assumed contact stress distribution method. For the impact problem we used a simple dynamic model based on quasi-static behavior of the panel—the sandwich beam was modeled as a combination of two springs, a linear spring to account for the global deflection and a nonlinear spring to represent the local indentation effects. Results indicate that the contact stiffness of the beam with graded core increases causing the contact stresses and other stress components in the vicinity of contact to increase. However, the values of maximum strains corresponding to the maximum impact load are reduced considerably due to grading of the core properties. For a better comparison, the thickness of the functionally graded cores was chosen such that the flexural stiffness was equal to that of a beam with homogeneous core. The results indicate that functionally graded cores can be used effectively to mitigate or completely prevent impact damage in sandwich composites.     

Indentation failure in composite sandwich structures

A combined analytical and experimental investigation of the indentation failure of a composite sandwich panel has been undertaken. Two cases have been studied: a sandwich panel with carbon/epoxy facing and a PVC foam layer supported on a rigid base and indented at the center with a cylindrical indentor; and a sandwich beam with symmetrical facing and core materials as in the sandwich panels. The load-deflection behavior of the loaded facing was monitored during the test. Strains were also measured near the load on both surfaces of the facing using embedded strain gages. A full-field analysis of the in-plane displacements in the foam was conducted using the moiré method. The problem was modeled as an elastic beam resting on an elastic-plastic foundation. Initiation of indentation failure occurs when the foundation yields, while catastrophic failure takes place when the compression facing fractures. The experimental results are in good agreement with the results of the analytical modeling based on the Winkler foundation.     

Comparison of slamming and whipping loads by fully coupled hydroelastic analysis and experimental measurement

This paper proposes a numerical method for analyzing whipping using a fully coupled hydroelastic model. The numerical analysis method utilizes a 3-D Rankine panel method, 1-D/3-D finite element methods, and a 2-D generalized Wagner model, which are strongly coupled in the time domain. The computational results were compared with those of a model test of an 18 000-TEU containership. The slamming pressures and whipping responses to regular waves for bow flare and stern slamming were compared. Furthermore, the slamming pressure was decomposed into its dynamic and static components. The numerical and experimental models produced similar results. In addition, the effects of the discretization and geometric approximation of the 2-D slamming sections were investigated.     

Debris impact forces on flexible structures in extreme hydrodynamic conditions

Field surveys of recent major flood events have emphasized the need for an in-depth examination of debris loading. Debris loading occurs when solid objects entrained within the flow interact with a structure in its movement path, exerting loads through direct impacts or damming. Until now, the focus of research into debris impacts has concentrated on single debris impacting a rigid structure. This study extends to examining the influence of debris impacting a flexible structure. A two degree-of-freedom (2DOF) system was used to model the debris–structure interaction. The model is compared with two experimental data sets using a rigid body and the effective stiffness model. The proposed 2DOF model was shown to be more accurate in estimating the impact force than those to which it was compared to. However, due to difficulties in estimating the stiffness and inertia of the structure and the debris, the proposed model under predicted the maximum measured impact loads. The effective stiffness model was shown to represent the maximum measured impact loads, and should therefore be implemented in the design process when considering debris impacts on flexible structures.     

Response and Damage Tolerance of Composite Sandwich Structures under Low Velocity Impact

The deformation and failure response of composite sandwich beams and panels under low velocity impact was reviewed and discussed. Sandwich facesheet materials discussed are unidirectional and woven carbon/epoxy, and woven glass/vinylester composite laminates; sandwich core materials investigated include four types of closed cell PVC foams of various densities, and balsa wood. Sandwich beams were tested in an instrumented drop tower system under various energy levels, where load and strain histories and failure modes were recorded for the various types of beams. Peak loads predicted by spring-mass and energy balance models were in satisfactory agreement with experimental measurements. Failure patterns depend strongly on the impact energy levels and core properties. Failure modes observed include core indentation/cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core. In the case of sandwich panels, it was shown that static and impact loads of the same magnitude produce very similar far-field deformations. The induced damage is localized and is lower for impact loading than for an equivalent static loading. The load history, predicted by a model based on the sinusoidal shape of the impact load pulse, was in agreement with experimental results. A finite element model was implemented to capture the full response of the panel indentation. The investigation of post impact behavior of sandwich structures shows that, although impact damage may not be readily visible, its effects on the residual mechanical properties of the structure can be quite detrimental.     

入水冲击问题研究的现状与进展

无论在理论分析还是数值模拟方面,入水冲击和砰击都是比较复杂的瞬态物理问题。按照时间历程分三个阶段综述了入水冲击理论、试验和数值仿真的发展概况,阐述了入水冲击和砰击现象现阶段研究的内容和热点,特别针对数值建模技术发展的研究状况进行重点叙述。为航天和航海领域进一步开展入水研究提供参考。     

Impact of a surfacewave on an elastic hull

In order to model a ship hull’s response to the impact of surface waves, the two-dimensional problem of wave impact on an elastic beam whose ends are connected by springs with a rigid structure uniformly submerged in a fluid is considered. The fluid is assumed to be ideal and incompressible and its flow symmetric; the lateral bending of the beam is described by the Euler equation. The fluid flow and the size of the wetted region are determined simultaneously with the calculation of the the beam deflection within the framework of the Wagner approach which takes into account the reshaping of the free surface of the fluid on interacting with a body. The stresses and strains arising in the beam and at its ends during impact are found. The numerical algorithm developed makes it possible to analyze the elastic effects in fluid impacts on thin-walled structures of finite length. Moreover, as the stiffness of the connecting springs tends to zero, the solution of this problem describes the impact of an elastic beam with free ends on a weakly curved fluid surface.     

泡沫铝夹层结构抗冲击性能的近场动力学模拟分析

针对某光学舱所采用的泡沫铝夹层防护结构在破片冲击下的抗冲击性能问题,采用Monte-Carlo方法创建了泡沫铝结构的二维细观模型,在常规态型近场动力学理论中引入了Mises屈服准则和线性各向同性强化模型,建立了近场动力学塑性本构的数值计算框架。基于近场动力学计算程序模拟了低速冲击作用下泡沫铝夹层结构的塑性变形以及有机玻璃背板的裂纹扩展形态,分析了泡沫铝芯材孔隙率对该夹层结构抗冲击性能和损伤模式的影响规律。结果表明：泡沫铝夹层结构良好的塑性变形能力是其发挥缓冲与防护作用的主要因素,并且在一定范围内,泡沫铝芯材孔隙率越高,则夹层结构具有更好的抗冲击性能;当泡沫铝孔隙率从0.4提升到0.7时,泡沫铝对冲击物的动能吸收率从90%提高到99%;模拟结果与实验结果具有较好的一致性,验证了模拟结果的准确性和分析结论的有效性。通过数值模拟,预测了有机玻璃背板的裂纹扩展形态,发现提高泡沫铝的孔隙率能获得更好的防护效果。     

刚体撞水非线性响应的边界元分析

考虑了非线性性液面条件液面条件,采用边界元法,始终撞水物体和水看成一个耦合系统,并作为一个冲击／接触问题处理。在自由注面的处理上,采用逐步迭代方法,使其始终满足液面的非线性条件,最终使自由液面达到一满足非线性条件的稳定值。而在撞水力的求解上,亦采用与自由液面相关的逐步迭代逼近方法,求取其稳定值。文中最后给出了几种不同情况下二维圆柱刚体撞水的计算实例,结果有力表明记叙虎法的可靠性和有效性。     

Free surface flow impact on a vertical wall: a numerical assessment

The sudden impact of a free surface flow upon a solid wall is a common occurrence in many situations in nature and technology. The design of marine structures is probably the most obvious example, but also river and dam hydraulics as well as the necessity of understanding flood and debris flow-induced damage have led to theoretical and experimental work on the mechanism of fluid slamming loads. This is therefore a very old and rich research field, which has not yet reached full maturity, so that semi-empirical methods in design practice are still the rule in many sectors. Up-to-date CFD technology with both Eulerian and Lagrangian approaches is employed to investigate highly non-stationary fluid impact on a solid wall. The development of the pressure wave produced by the impact is examined as it propagates and interacts with the fluid boundaries, as well as the subsequent build-up of high-pressure gradients of high fluid velocities. The geometry and the velocity field of the problem considered are very simple, but the results seem to provide new insight, in particular, into the connection between phenomena with different timescales.